In recent years, mobile traffic keeps increasing due to growing popularity of smartphones and the like. To address the increase in the mobile traffic, for example, studies are in progress on Massive Multi Input Multi Output (MIMO) systems in which a beam is formed by using a large number of antenna elements. When a large number of antenna elements are used, in addition to an advantageous effect where the capacity of a network is increased, another advantageous effect is also expected where it is possible to form a narrow beam and to consequently reduce interference occurring among adjacently-positioned cells.
Further, studies are also conducted on high-density distributed antenna systems in which a plurality of antenna elements connected to one baseband processing apparatus are arranged in multiple locations in a distribute manner so as to increase the capacity of the network. In such high-density distributed antenna systems, when the number of the antenna elements arranged in each location in a distributed manner is small, it is difficult to form a narrow beam in each location, and there is a possibility that interference may occur among adjacently-positioned locations. However, because all the antenna elements are connected to the one baseband processing apparatus, it is possible to reduce the interference occurring among the locations by exercising cooperative control over the antenna elements arranged in the various locations.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2015-8530
Japanese National Publication of International Patent Application No. 2012-510219
Incidentally, to increase the capacity of a network while signals are transmitted from a plurality of antenna elements, mutually-different signals are transmitted simultaneously from mutually-different antenna elements. In other words, for example, in a base station apparatus adopting the Massive MIMO system, the signals transmitted from mutually-different antenna elements have a phase difference. Consequently, the signals simultaneously transmitted through the antenna elements are not the same as one another.
For this reason, in a wireless communication system in which a plurality of wireless stations each including a plurality of antenna elements are connected to one baseband processing apparatus, the number of the antenna elements usable in a transmission from each of the wireless stations may be limited in accordance with the amount of data that can be transmitted and received between the baseband processing apparatus and the wireless station. In other words, when the amount of data received from the baseband processing apparatus is large, the number of the antenna elements that are simultaneously usable by the wireless station is set to a large value. On the contrary, when the amount of data received from the baseband processing apparatus is small, the number of the antenna elements that are simultaneously usable by the wireless station is set to a small value.
In this manner, the number of the antenna elements usable by each of the wireless stations is limited in accordance with the amount of data transmitted and received between the baseband processing apparatus and the wireless station. Accordingly, there is a case where the interference among the wireless stations is not sufficiently reduced, depending on resources allocated to the wireless stations. More specifically, when not many resources are allocated to a wireless station, because the amount of data transmitted to the wireless station is small, the number of the antenna elements that are simultaneously usable is small. As a result, the wireless station is not able to form a sufficiently narrow beam. Consequently, for example, the levels of interference increase at the terminal devices other than the terminal device to which the signal is transmitted.
In particular, it is difficult to exercise cooperative control over wireless stations connected to mutually-different baseband processing apparatuses. Accordingly, there is a possibility that interference may increase among adjacently-positioned wireless stations in the vicinity of a boundary between two baseband processing apparatuses.